Electrochemical Degradation with Real Textile Effluent with UV Using Platinum-plated Titanium Anode
Article information
Abstract
In order to investigate the performances of electrochemical degradation with ultraviolet (UV) at 254 nm for real textile effluent, a cylinder electrolyzer with a titanium cylinder cathode was adopted to optimize initial pH, flow rate, initial Na2SO4 concentration, degradation time and cell voltage with a commercial UV lamp at 254 nm located in the center of the platinum-plated titanium cylinder anode. The experimental results illustrated that the COD of textile effluent could reduce from 120.4 mg L−1 to 80.3 mg L−1 with corresponding current efficiency of 91.60%, electrical consumption of 34.1kWh kg−1 and carbon emission of 19.4 kg CO2 kg−1 COD by electrochemical degradation with UV at 254 nm at degradation of 10 min. Additionally, degradation performances of real textile effluent by electrochemical degradation with UV at 254 nm was more effective than those of electrochemical degradation alone and UV at 254 nm alone. Therefore, electrochemical degradation with UV at 254 nm was promising for the degradation of real textile effluent in the industrial application.
1. Introduction
Nowadays, a large amount of textile effluent was generated in the textile industry. Since textile effluent with organic contaminants is harmful to ecological environment and human health [1], many treatment methods have been investigated, such as adsorption, membrane separation, ultraviolet (UV) and electrochemical degradation. Treatment and regeneration of adsorbents for textile effluent still require further research [2]. As for the membrane separation, it still faces the problems of membrane fouling and membrane pollution [3].
UV could activate H2O2, chlorine and persulfate, which could improve the degradation performance of the wastewater. UV treatment for the wastewater mainly consists of UV at 185 nm and UV at 254 nm [4,5]. It has been revealed by related research that ozone can be generated by UV radiation at 254 nm with O2 absorbed in UV lamp [6]. In addition, some research suggested that UV at 254 nm could degrade some organic contaminants by its electricity [7]. The UV treatment generally was adopted with the addition of oxidant chemicals.
Electrochemical degradation of organic contaminants mainly based on hydroxy radical and sulfate radical has been widely researched [8,9]. Electrolyzer structure and electrode material are the main research aspects in electrochemical degradation [10,11]. Plate electrodes are widely adopted in the electrochemical degradation of wastewater. Cylinder electrodes can result in more sufficient mass transfer and more compact electrolyzer structure compared with plate electrodes. The widely investigated electrode materials include boron-doped diamond (BDD) electrode, PbO2 electrode and ruthenium-iridium electrode [12]. Besides, persulfate ion can be generated by the oxidation of sulfate ion using platinum as the anode. Compared with platinum, platinum-plated titanium electrode as the anode obtains much research in electrosynthesis of persulfate because of the lower price of platinum-plated titanium [13]. However, it lacks the study in the electrochemical degradation of textile effluent by platinum-plated titanium electrode.
Electrochemical degradation with UV has been reported as a more effective method to degrade organic contaminants [14]. High removal efficiency of p-nitrophenol was achieved by 30 min treatment with the high salinity by electrochemical process coupled with UV at 254 nm [15]. In addition, the study on the synergistic mechanism of enhanced degradation performance illustrated that chlorine radical with high oxidation capacity played an important role in the p-nitrophenol oxidation. However, the addition of chlorine salt could generate chlorine gas that is harmful to the human health. To our best knowledge, it lacks the research in the electrochemical degradation with UV at 254 nm to achieve the generation of the persulfate on the platinum-plated titanium electrode and the generation of sulfate radical by the activation of UV at 254 nm at the same time.
Carbon neutrality has obtained more and more attention in wastewater treatment [16]. In addition, the accounting of carbon emissions plays a significant part in the carbon neutrality, which has obtained much attention in the treatment of textile effluent [17]. Therefore, as for the electrochemical degradation of textile effluent with UV, carbon emissions would be a research focus [18]. However, it still lacks related research in carbon emissions for the treatment of textile effluent by electrochemical degradation with UV.
In this work, electrochemical degradation with UV at 254 nm using platinum-plated titanium anode was proposed to degrade the real textile effluent after biochemical treatment with COD, current efficiency, electrical consumption and carbon emissions investigated. The working parameters were systematically investigated in a cylinder electrolyzer with platinum-plated titanium mesh cylinder anode and titanium cylinder cathode, including initial pH, flow rate, initial Na2SO4 concentration, degradation time and cell voltage. The performances of electrochemical oxidation with UV, electrochemical degradation alone and UV alone were compared to provide more basic information for the efficient treatment of the textile effluent.
2. Experimental
2.1 Materials and chemicals
Sulfuric acid (H2SO4), sodium hydroxide (NaOH) and sodium sulfate (Na2SO4) were of analytical grade. The textile effluent after biochemical treatment was obtained from a textile enterprise in Zhejiang province. The commercial UV lamp at 254 nm (GHO36T5H/80W(UK)) was purchased from Suzhou Hemingway Environmental Protection Equipment Co., Ltd. The platinum-plated titanium mesh electrode was purchased from Baoji Shuangliheng Titanium Technology Co., Ltd. The cylinder electrolyzer were purchased from Shaanxi Utron Environmental Protection Technology Co., Ltd.
2.2 Experimental apparatus
As shown in Fig. 1, the UV lamp with the diameter of 254 mm, the length of 843 mm and UV intensity of 320 μm cm−2 was located in the center of the platinum-plated titanium mesh cylinder anode with the diameter of 40 mm and the height of 680 mm. Platinum with the thickness of 0.5 mm was electrodeposited on titanium cylinder with the mesh of 5. The cathode was the titanium cylinder with the diameter of 70 mm, the thickness of 2 mm and the height of 680 mm. The space between the anode and the cathode was 13.5 mm. A magnetic pump (MP-55R, Baoding Qili Precision Pump Co., Ltd) and a DC power supply (JK3060K, Shenzhen Junke Instrument Technology Co., Ltd) were connected with the cylinder electrolyzer.
2.3 Experimental design
As shown in Fig. 1, the textile effluent of 25 L continually flowed between the cylinder electrolyzer and the effluent reservoir by pump during experiments. The experiments of electrochemical degradation with UV were carried out with the original textile effluent at the constant cell voltage when the DC power supply and the UV lamp were linked to electricity. To further evaluate the effectiveness of the electrochemical degradation with UV for textile effluent, the experiments of UV alone and electrochemical degradation alone were conducted. When the experiments of electrochemical degradation alone were performed in the cylinder electrolyzer, the UV lamp cut off. The experiments of UV alone were performed when the DC power supply cut off. 0.1 mol L−1 H2SO4 solution and 0.1 mol L−1 NaOH solution were used to adjust the pH of textile effluent with initial pH of 7.6. The experiments were operated during the temperature from 30°C to 35°C. The analysis samples were extracted from the effluent reservoir when each experiment was finished. The UV-visible spectrums of the samples were measured by an ultraviolet-visible spectrophotometer (UV-2600, Shimadzu, Japan). The changes of functional groups and chemical compositions of textile effluent were analyzed by in infrared spectrometer (8400S, Shimadzu, Japan).
2.4 Calculation
The COD of textile effluent was measured by a spectrophotometer (DR3900, Hach, USA). Current efficiency (η, %) that presents the utilization rate of the charge during electrochemical degradation, is calculated using Eq. 1.
where CODo and CODt are the COD of the textile effluent before degradation and after degradation at time of t, respectively, mg L−1; V is the volume of textile effluent, 25 L; F is the Faraday constant, 96485 C mol−1; I is the current of the power supply, A; t is the time when the current is supplied, s.
Since the electrical consumption consists of cylinder electrolyzer, UV lamp and magnetic pump, the electrical consumption per kilogram of COD (W, kWh kg−1) is calculated using Eq. 2:
where U is the cell voltage of the cylinder electrolyzer, V; Pu is the power of the UV lamp, 80 W; Pm is the power of the magnetic pump, 15 W.
The carbon emissions are due to the electrical consumption in present work. The carbon emissions per kilogram of COD (We, kg CO2 kg−1 COD) is calculated using Eq. 3:
where f is the carbon emissions factor in China, 0.5703 kg CO2/kWh.
3. Results and Discussion
3.1 Comparison of different degradation methods
Fig. 2–4 displays the degradation performances of the textile effluent by different degradation methods with initial pH of 5.0, flow rate of 120 L h−1, initial Na2S2O8 concentration of 0.1 mol L−1 and degradation time of 10 min. In addition, the electrochemical degradation with UV (EU+UV) and electrochemical degradation alone (EU) were performed at cell voltage of 5.0 V. It can be observed from Fig. 2 that there existed the aromatic structure was characterized by the peaks at 281–285 nm [19] in the original textile effluent. The peaks at 281–285 nm presented a rapid reduction after electrochemical degradation or UV. Compared with electrochemical degradation alone (EU) and UV alone, the peak exhibited more reduction by electrochemical degradation with UV (EU+UV), which the demonstrated that the good degradation performances of the textile effluent by electrochemical degradation with UV using platinum-plated titanium anode.

Degradation performance of different treatment methods initial pH of 5.0, flow rate of 120 L h−1, initial Na2S2O8 concentration of 0.1 mol L−1, degradation time of 10 min and cell voltage of 5.0 V.
As shown in Fig. 3, the peak at 3446 cm−1 and 617 cm−1 was due to the –OH stretching vibration of hydroxyl groups and C–Cl stretching vibration, respectively [20]. The peak at 875 cm−1 was due to the vibration of C=C bending [21]. A strong peak was observed at 1127 cm−1, which belonged to S–O band [22]. The peak at 1477 cm−1 was caused by the presence of –CH2 [23]. The peaks vibration of N=C=N stretching of carbodiimidd appeared at 2120–2176 cm−1 and the peak at 2010 cm−1 was attributed to the vibration of C≡C stretching alkyne in the FTIR spectras of EU+UV and EU. It can be observed that the functional groups and chemical compositions of textile effluent after UV treatment almost were same as original textile effluent. The peaks at 875 cm−1 and 1477 cm−1 disappeared after electrochemical degradation with UV (EU+UV) and electrochemical degradation alone (EU) compared with original textile effluent, which indicated that the chemical compositions with C=C got removed effectively after electrochemical degradation with (EU+UV) and electrochemical degradation alone (EU).
As shown in Fig. 4, the COD after electrochemical degradation with UV (EU+UV), electrochemical degradation alone (EU) and UV alone (UV) were 80.3 mg L−1, 95.3 mg L−1 and 110.9 mg L−1 with the corresponding electrical consumption of 34.1 kWh kg−1, 33.2 kWh kg−1 and 66.7 kWh kg−1, respectively. It was obvious that the degradation performances of electrochemical degradation with UV (EU+UV) were better compared with UV alone (UV). On one hand, the organic contaminants could be electrochemically degraded on the surface of platinum-plated titanium anode under electrical voltage. On the other hand, the persulfate radical could be generated on the platinum-plated titanium anode under electrical voltage shown in Reaction (1) with the oxygen evolution shown in Reaction (2). Persulfate radical as an oxidizing ion could achieve the degradation of organic contaminants. Therefore, the direct degradation of organic contaminants and the generation of persulfate radical were the main effects of electrical voltage. As for the UV lamp, persulfate ion could be activated by UV at 254 nm with sulfate radical (SO4−) generated shown in Reaction (3) [24]. The SO4− could achieve the degradation of textile effluent. Considering the degradation performance of UV for textile effluent to a certain extent shown in Fig. 4, activation of persulfate ion and degradation of organic contaminants were main effects of UV lamp.
Although the electrical consumption of electrochemical degradation alone (EU) with quite high COD reduction was lower than that of electrochemical degradation with UV (EU+UV), activation of persulfate ion and degradation of organic contaminants by UV lamp improved the COD reduction of electrochemical degradation with UV (EU+UV) compared with electrochemical degradation alone (EU), which achieved the higher COD reduction than that of electrochemical degradation alone (EU) plus UV alone (UV). Therefore, electrochemical degradation with UV (EU+UV) adopting UV lamp has better degradation performances of textile effluent than that of electrochemical degradation alone (EU).
The activation of persulfate ion and degradation of organic contaminants could be enhanced with the increase of intensity of UV lamp [25]. Intensity of commercial UV lamp at 254 nm was proportional to the length of UV lamp and the longest UV lamp was utilized in the present cylinder electrolyzer. Therefore, the degradation performance of electrochemical degradation with UV was not investigated in different intensity of UV lamp in this work.
3.2 The effect of initial pH and flow rate on degradation performances
Fig. 5 presents that COD and current efficiency under different initial pH at flow rate of 120 L h−1, initial Na2S2O8 concentration of 0.1 mol L−1, degradation time of 10 min and cell voltage of 5.0 V. As shown in Fig. 5, the COD reduced at first and then gradually increased when the current efficiency increased slightly at first and then gradually reduced. The solution conductivity was decreased due to the reduction of corresponding added H2SO4 concentration as the initial pH increased from 4.0 to 5.0, which lowered the current at the same cell voltage and increased the current efficiency according to Eq. 1. In addition, the reduction of H2SO4 concentration lowered the persulfate generation in Reaction (1), which caused the increase of COD from 80.3 mg L−1 to 87.2 mg L−1 and the reduction of current efficiency from 91.60% to 87.81% at initial pH from 5.0 to 7.0. As the initial pH increased from 7.0 to 9.0, the generation of persulfate ion could be lowered by the alkaline as shown in Reaction (4) [26], which caused the decrease of COD from 87.20 mg L−1 to 90.40 mg L−1. The increase of COD and solution conductivity were attributed to the drop of current efficiency from 87.81% to 73.54% at initial pH from 7.0 to 9.0. Considering the higher current efficiency and the lower COD, initial pH of 5.0 was selected for the following experiments.

COD and current efficiency under different initial pH at flow rate of 120 L h−1, initial Na2S2O8 concentration of 0.1 mol L−1, degradation time of 10 min and cell voltage of 5.0 V.
As shown in Fig. 6, the COD was gradually reduced while the current efficiency was gradually increased with flow rate from 80 L h−1 to 130 L h−1 at initial pH of 5.0, initial Na2S2O8 concentration of 0.1 mol L−1, degradation time of 10 min and cell voltage of 5.0 V. As flow rate increased from 80 L h−1 to 120 L h−1, the mass transfer between the textile effluent and the anode was gradually enhanced, which resulted in the reduction of COD and the increase of current efficiency. When the flow rate reached 120 L h−1, the mass transfer was sufficient in the cylinder electrolyzer, which caused that COD and current efficiency remained relatively stable at flow rate from 120 L h−1 to 130 L h−1. Therefore flow rate of 120 L h−1 was selected for the following experiments.
3.3 The effect of initial Na2SO4 concentration and degradation time on degradation performances
As shown in Fig. 7, the COD gradually decreased while the current efficiency was increased at first and then gradually reduced with initial Na2SO4 concentration from 0 to 0.5 mol L−1 at initial pH of 5.0, flow rate of 120 L h−1, degradation time of 10 min and cell voltage of 5.0 V. With the increase of initial Na2SO4 concentration, the generation of persulfate ion was enhanced, which improved the degradation of organic contaminants in textile effluent and reduced the COD. As the initial Na2SO4 concentration increased from 0 to 0.1 mol L−1, the reduction of COD was attributed to the increase of current efficiency from 84.42% to 91.60%. When the initial Na2SO4 concentration was beyond 0.1 mol L−1, the solution conductivity was further increased due to the further increase of initial Na2SO4 concentration, which resulted in the increase of electricity and the reduction of current efficiency at the same cell voltage according to Eq. 2. Although the COD at initial Na2SO4 concentration of 1.0 mol L−1 was higher than that at initial Na2SO4 concentration beyond 0.1 mol L−1, the current efficiency at initial Na2SO4 concentration of 1.0 mol L−1 was much higher than that at initial Na2SO4 concentration beyond 0.1 mol L−1. Therefore, initial Na2SO4 concentration of 0.1 mol L−1 was selected for the following experiments.

COD and current efficiency under different initial Na2SO4 concentration at initial pH of 5.0, flow rate of 120 L h−1, degradation time of 10 min and cell voltage of 5.0 V.
Fig. 8 displays the degradation performances under degradation time from 10 min to 60 min at initial pH of 5.0, flow rate of 120 L h−1, initial Na2S2O8 concentration of 0.1 mol L−1 and cell voltage of 5.0 V. It could be observed from Fig. 8 that the COD and the current efficiency decreased with the increasing degradation time. On one hand, the accelerated degradation time resulted in the increase of degraded organic contaminants in the textile effluent, which was attributed to the reduction of COD. On the other hand, as degradation time increased from 10 min to 60 min, more byproducts were generated with less degradable organic contaminants that could hinder the mass transfer between the organic contaminants in textile effluent and the anode, which resulted in the reduction of current efficiency from 91.60% to 38.97%. As shown in Fig. 8, the COD at degradation time of 60 min reduced to 54.1 mg L−1, which met the first-order discharge standard for municipal wastewater treatment plant in China [27].
3.4 The effect of cell voltage on degradation performances
Fig. 9 shows the degradation performances under cell voltage from 4.0 to 6.0 at initial pH of 5.0, flow rate of 120 L h−1, initial Na2S2O8 concentration of 0.1 mol L−1 and degradation time of 10 min. It can be seen from Fig. 8(a) that the COD gradually reduced and the current efficiency increased at first and then reduced. With the increase of cell voltage from 4.0 to 6.0, the direct electrochemical oxidation of organic contaminants on the anode and the persulfate ion generation shown in Reaction (1) were enhanced, which improved the degradat ion of organic contaminants and the reduction of COD from 97.3 mg L−1 to 71.6 mg L−1. The increase of COD reduction contributed to the increase of current efficiency at cell voltage from 4.0 to 5.0 as shown in Fig. 8(a) according to Eq. 2. However, with the further increase of cell voltage from 5.0 to 6.0, the oxygen evolution reaction shown in Reaction (2) on the anode was enhanced, which lowered the effective reaction surface of the anode and caused the reduction of current efficiency.

Degradation performances under different cell voltage: (a) COD and current efficiency, (b) electrical consumption and carbon emissions at initial pH of 5.0, flow rate of 120 L h−1, initial Na2S2O8 concentration of 0.1 mol L−1 and degradation time of 10 min.
As shown in Fig. 9(b), electrical consumption and carbon emissions decreased at first and then increased. With the increase of cell voltage from 4.0 V to 5.0 V, the enhancement of direct electrochemical oxidation of organic contaminants on the anode and persulfate ion generation shown in Reaction (1) improved the reduction of COD and the increase of current efficiency, which was attributed to the decrease of electrical consumption from 43.6 kWh kg−1 to 34.1 kWh kg−1. When the cell voltage was beyond 5.0 V, the enhancement oxygen evolution reaction shown in Reaction (2) on the anode lowered the current efficiency, which caused the increase of electrical consumption from 34.7 kWh kg−1 to 37.6 kWh kg−1 according to Eq. 2. It could be seen that the change tendency of carbon emissions with the increase of cell voltage was consistent with electrical consumption because the carbon emissions were calculated by electrical consumption multiplied by the carbon emissions factor according to Eq. 3. The carbon emissions was 19.4 kg CO2/kg COD at cell voltage of 5.0 V. In addition, it could be calculated that the carbon emissions by the electrical consumption of electrochemical degradation alone and UV lone were 10.4 kg CO2 kg−1 COD and 7.6 kg CO2 kg−1 COD, respectively, which were the main sources of carbon emissions in the present experiments of electrochemical degradation with UV. Consequently, the optimization of experimental parameters for electrochemical degradation with UV was an effective method to reduce the carbon emissions.
4. Conclusions
Degradation performances of real textile effluent by electrochemical degradation with UV at 254 nm was more effective than those by electrochemical degradation alone and UV at 254 nm alone. As for initial textile effluent COD of 120.4 mg L−1, the COD could reduce to 80.3 mg L−1 with current efficiency of 91.60%, electrical consumption of 34.1 kWh kg−1 and carbon emissions of 19.4 CO2/kg COD by electrochemical degradation with UV at initial pH of 5.0, flow rate of 120 L h−1, initial Na2S2O8 concentration of 0.1 mol L−1, degradation time of 10 min and cell voltage of 5.0 V. In addition, the COD could reduce to 54.1 mg L−1 at degradation time of 60 min, which met the first-order discharge standard for municipal wastewater treatment plant in China. The experimental results suggested that the degradation of real textile effluent using electrochemical degradation with UV at 254 nm would be an effective method in the industrial application.
Notes
ACKNOWLEDGEMENTS
This study was financially supported by the Postdoctoral Foundation of Zhejiang Sci-Tech University Tongxiang Research Institute (grant number: TYY202301).
CONFLICTS OF INTEREST
The authors declare no conflict of interest.